Method of evacuating



- 'May 10,1932. YK.CI.D.IHIC KMAN 7, 8

METHOD or 'EVAC'UATING Filed April 4, 1931 Elm wow,-

116 m 01). Hid/11mm, I

Patented May 10 1932 UNITED STATES? PATENT OFFICE KENNETH o. 1). 11mm, or nocnnsrnn, mew Yonx, assmnoa 'ro mam xonnx comrm, or nocnasmna, NEW YORK, A conromvrron on NEW YORK mnon or nvncua'rme I Application filed a rn i, 1931. Serial no. 527714.

My present invention relates to a method of evacuating an enclosed space by means of organic liquids of low volatility and decomposition rate, such .as the esters of phthalic acid. The organic liquid is evaporated by heat and thefvapors are passed through a jet thus ejecting them from the evaporating flask at high velocity. These vapors envelop or enclose the gases with which they contact and carry-them along away from the point of contact, thus continually removing the gases from a certain area. By attaching the closed vessel to be evacuated to this area with which the current of organic vapor contacts, removal of the gases contained therein and consequent diminution of pressure is accomplished. The vapors are in practice condensed and returned to the evaporating flask to diminish back ressure which might seriously impair the et ciency of the process.

The present application is in arta continuation of my application erial No. 341,105 :filed February 19, 1929.

In the past in the various types of vacuum pumps which-have relied on a current of vapor as the evacuating agent, mercury has been employed for the reason that it is an elementary substance and consequently will not break down into gaseous or objectionable compounds. Also its position in the electromotive series is indicative of its resistance to acidic materials. However mercury has a high surface tension and when it condenses it gathers together inglobules or droplets instead of distributing ltself over the condensing surface. Consequently when the atoms of the mercury vapor hit the condensing surface some of the mercury which hits the surface not wett'ed b aliquid will deflect or bounce from the surflice before condensation can take place. This has the effect that the total area of the condensing surface is not concerned.

effective in inducing condensation;

Mercury has another disadvantage in the vapor ty e vacuum pumps, in that the-gases carried orward b the vapor stream may leak backwards between globules of the vapor rendering the pressure on the hi h vacuum side of the pump a definite function of that on the lower side, instead of, letting it be I within limits, independent thereof. In the absence of an effective backing pump to co operate with the vapor-type pump when it contains mercury, the effectiveness of the vapor-type evacuation pump is considerably reduced.

. A further disadvantage of mercury is that Its vapor pressure at ordinary temperatures may be as high 'as a micron (10' m. m.) of mercury so that it is necessary to'interpo'se a low temperature trap to prevent diffusion of vapor from the pump to the vessel being evacuated, when the highest vacuums 'are desired. An example of a low-temperature trap that may be used is a U-tube cooled by liquid air.

I have found that organic liquids having certain characteristics are particularly suity 'able for employment in the vapor-type vacuum pump. The 'diflicul'ty volatile organic liquids which I have found especially suitable are the esters of the hthalic acids such as n-butyl phthalate, 1so amyl 'phthalate,

' n-butyl terephtha1ate,n-amyl phthalate,butyl benzyl phthalate or like esters of phthalic acid. The organic liquids that have been found mostsuitable in this connection are the phthalic, isophthalicor terephthalic acid esters of the methyl, ethyl, propyl, butyl, amyl or ,benzyl alcohols either normal or isomeric. The-characteristics these suitable liquids possess are: 4 j I (1) A vapor pressure at about 0 C. which is less than. thatencountered in industrial. high vacuum work.

. (2) The property of condensing easily as a film on a cooled surface.

(3) They may be boiled for long periods without chemical decomposition. I

(4) They are chemically'inert inso far as the material of the pump and the gases, whichare usually met'with in vacuum work,-are

I have found that the many disadvantages. of mercury are overcome to a in e extent by using certain high boiling organic liquidsin vapor-type vacuum pumps in place of mer- I:

cury. Although low vapor .pressure is the most important criterion of the suitability more volatile constituents.

' trap previously referred to.

' tension and will wet the of a liquid, obviously high boiling liquids which are decomposed by boiling are not adapted to be employed in this connection. Certain aliphatic liquids such as thehigher and the long chain acids and esters, both saturated and unsaturated, although they do eventually produce high vacua whenused in vapor ty e vacuum pumps, do so com- .paratively slow probably because of the cracking of these compounds by heat, into If the cracking of these com ounds is the cause of diminution, of spee of evacuation, the reason for this seemmgly lowered efliciency is apparently due tothe fact that the pump must perform the double duty of removing the gas from the .vessel being evacuated and also of removing the more volatile waste products of the cracked vapor.

On the other hand, the esters of the phthal ic acids which I employ'have a sufliciently low vapor pressure at room temperature that they may even be used without a liquid air These liquids have a low surface condensing. surface so that this surface is effective over its whole area in inducing condensation and a shorter pump maybe .used as the vapor condenses within a well defined zone. This behavior is in contrast with that of mercury. When mercury is employed its vapor wanders out of the'pump and is found deposited in various corners and bends. In the vacuum pumps employing mercury it has been necessary to provide a condensing system of relatively great length to lessen the )quantity of mercury vapor that might enter the vessel which is being evacuated.

However, the method which I have invented may be,carried out in any compatible type of vapor vacuum pump such as, for example, the pump disclosed in U. S. Patents Nos. 1,320,874 and 1,393,550 of Langmuir or in the pumps disclosed in my co-pending application No. 341,105. 'As a specific example of the carrying out of this "process, it is described with reference to the vacuum pump shown in Figure I which is claimed in my co-lpending application No. 341,105.

eferring to the drawings,'.n which like reference characters refer to'like parts, Figure I is one form of evacuation pump in which my process may be carried out. Figures II,

' III, and IV are cross-sectional views of the pump depicted in Figure I, taken on lines 22, 3-3 and 4-4 thereof, respectively.

Referring to the drawings which illustrate apparatus suitable for carrying out my process, Figure I showsa pump which comprises a vessel 40 containing the organic liquid such as an ester of phthalic acid, 41, which is to be boiled'by any suitable source of heat applied thereto. The vessel 40 is constricted into a 'et 42 connected with-a reentrant'bulb 43 which is distorted into the form of an ex-i aggerated alembic which is connecte condensing tube 44. Bulb 43 is also con-. nected to the vessel to be evacuated through tube 47. i

The condensing tube 44 may be cooled by means of a radiator 45in the form of a metal collar provided with metallic radiating fins engagingthe tube 44 as shown in Figure II.

A heat radiator, 46 may also be rovidedaround bulb, 43, preferably forme arate fins such-as that shown in Figure III. The tube, 47, may also be provided with a coolin device. The cooling device may consist o spaced turns of wire, 48, as shown in detail 1n Figure IV. 1

I The space between jet, 42, and the bulb 43 is packed with a heat Insulating material, 51, to prevent transfer of heat from the jet tothe bulb or chamber. Also the condensing tube v44 is connected to the atmosphere preferably with a backing pump, 49, interposed.

In the operation, of my process, the organic li uid, 41, which may be butyl phthalate is boile and a stream of its vapor rapidlypasses out through the jet 42 into condensing tube 44. The current of vapor which passes from jet 42 into tube 44 entraps the air or gas which is present in bulb, 43. By this m'eans the pressure in bulb 43 is continuously lowered, which simultaneously lowers the pressure of the entire enclosed system represented by tube 47.

When the vapor reaches the tube, 44, it'condneses as a film, and drops back into bulb, 43, where it cools and collects and then passes throu h return tube, 50, into the container, 40. ere it is heated and again goes through the cycle of the operation.

With the process of, my application a 24 Y p t acle to obtain a high vacuum 'therein which comprises removing a portion of the entrained gases in the vessel to be evacuated by means of a current of phthalicacid ester vapors. y

2. A method of evacuating'a closed receptacle to obtain a high vacuum thereinv which comprises removing a portion of the entrained ases in. the closed receptacle .by means of t e'vapor of an ester of phthalic. acid selected from the group"';'consistin of normal-butyl'phthalate, iso-amyllphtha ate, normal-butyl terephthal'ate," normal-amyl phthalate,and.butyl benzylphthalate.

3. The method. of evacuating a closed .re-

of sepof the individceptacle to obtain a high vacuum therein which comprises removmg a part of the entrained ases in the closed receptacle by means of t e vapor of normal-butyl phthal ate.

4. The method of evacuating a closed receptacle to obtain a high vacuum therein I which comprises removing a part of the en- A trained gases in the closed receptacle by means which comprises va orizing an ester' of v phthalic acid selected rom the group consistmg of normal-butyl phthalate, iso-amyl phthalate, normal butyl terephthalate, normal-amyl phthalate and butyl benzyl phthalrating the condensed vapor and introducing ate, pro ecting a stream ofsaid vapors through a restricted jet connected with the receptacle to be evacuated, condensing the vapor at a remote point, re-evaporating the condensed 12. A means for evacuating closed vessels which comprises a difi'usion and condensat on vacuum pump containing n-butyl phthalate as an evacuating medium.

Signed at Rochester, New York, this 25th day of March, 1931.

KENNETH C. D. HICKMAN.

vapor andintroducing it into the moving stream of vapor.

7. The method of evacuating a closed receptable to obtain a high vacuum therein which comprises vaponzing normal-but l phthalate, projecting a stream of sand vapors through a restricted jet connected with the receptacle to be evacuated, condensing the vapor at; a remote point, re-evapoit into the moving stream of vapor.

8. The method of evacuating a closed receptacle to obtain a high vacuum therein which comprises vaporizing iso-amyl phthalate, projectin a stream of said vapors through a restricte jet connected with the receptacle to be evacuated, condensing the vapor at a remote point, re-evaporating the condensed vapor and introducing it into the movir ig stream of vapor.

9. he method of evacuating a closed receptacle to obtain a hi h vacuum therein which comprises vaporizing butyl benzyl phthalate, projectin a stream of said vapors through a restricte receptacle to be evacuated, condensing the vapor at a remote point, re-evaporating the condensed vapor and introducing it into the moving stream of vapor.

10. A means for evacuating closed vessels I .which comprises a difiusion and condensation vacuum pump containing an. ester of a phthalic acid as an evacuating medium. i 11. A means for evacuating closed vessels which comprises a diffusion and condensation "vacuum pump containing an alkyl phthalate 56 as an evacuating medium.

jet connected with the 

